Tutor a nine-year-old in fractions and you’ll find out fast which parts you only pretended to get.

Teaching something exposes every place you only half learned it. Explaining fractions to a nine-year-old means you cannot lean on the trick that carried you through the test, because they will ask why, and why is where the gap sits. That is uncomfortable and extremely useful. Nothing else finds the soft spots in your understanding so fast.

Pick a topic you feel shaky on and explain it out loud to somebody who does not know it. A sibling, a friend, an empty room, any of them work. Wherever you stall or start waving your hands is exactly what to study next.